Depending on their production systems and markets, vegetable growers can grow crops that either occupy an entire growing season or a few months of the growing season. In a late-planted fall vegetable production system where no other cash crop occupies the soil for the preceding part of the season, weeds can grow and contribute large amounts of seeds to the soil weed seed bank (Kumar et al., 2009). During this summer fallow season, many midwestern U.S. vegetable growers manage weeds by means of tillage or herbicides. Although effective, herbicides and excessive tillage lead to environmental issues such as herbicide-resistant weeds (Chatham et al., 2015) and reduced soil organic matter (Reicosky et al., 1995). Cover crops are an alternative to tillage and herbicides and offer additional benefits to the soil and environment while also increasing or maintaining vegetable yield (Kumar et al., 2009). Studies have reported successful weed suppression using cover crops (Bugg and Dutcher, 1989; Kumar et al., 2009) while also improving yield of the successive vegetable crop (Sainju et al., 2002). Observed mechanisms of weed suppression reported are weed seed reduction (Kumar et al., 2009), allelopathy (Rueda-Ayala et al., 2015), and reduction of weed seed germination following cover crop soil incorporation (Kumar et al., 2009).
Not only can cover crops be a suitable alternative to herbicides and tillage but also can provide other environmental benefits. Soil organic matter is one aspect that cover crops can influence in a positive way (Mukherjee and Lal, 2015). Ding et al. (2006) reported increased soil organic matter in soils with cover crops compared with a no-cover crop (control). Cover crops also benefit pollinators (Clark, 2007), improve soil fertility (Teasdale, 1996), and increase soil microbial biomass (Buyer et al., 2010). Corak et al. (1991) and De Baets et al. (2011) reported reduction in soil erosion especially with fibrous rooted cover crops such as common oats (Avena sativa).
Vegetable growers in the midwestern United States know the importance and relevance of cover crops in their cropping systems but are hesitant to use cover crops, especially in the summer. The variable climate and narrow seasonal window for growing vegetables in Iowa demands maximum use of the growing period to grow cash crops, thus innovative strategies have to be developed for integration of cover crops and cover crop mixtures in vegetable cropping systems. Often there is a 4 to 6 week summer fallow between spring/summer harvested and fall planted vegetable crops. This period could benefit from a cover crop but growers need cover crops that would grow in that short time and provide benefits, but at the same time are easy to terminate and do not harm the successive vegetable crop. Few studies have experimented with summer cover crops and their influence on vegetable crop production (Creamer and Baldwin, 2000; Wang et al., 2008a). More information and research is needed on cover crop type, the biomass they generate, and the effects they have on soil properties and succeeding vegetable crops. Additionally, appropriate planting time, after cover crop soil incorporation, needs to be evaluated, since negative impacts of cover crops on succeeding vegetables have been reported (Ackroyd and Ngouajio, 2011).
The primary objective of this study was to identify appropriate summer cover crops and investigate their effects on fall lettuce production in the midwestern United States. The study also evaluated the interval between cover crop incorporation and lettuce planting to determine the right planting time. This information can help growers identify a suitable cover crop that can provide weed suppression during fallow and improve soil properties without detrimentally affecting lettuce growth and yield.
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